Originally Posted by MrWicked
the rear axle code is C2
and your thoughts on the 4:1- gear with a 31" tall tire???/
That's great you found with with factory limited slip. Seems VERY rare on the used market. It's seem that when people order a useful configuration, they never sell it, where as the fleets all like ordering the 5.4L w/ 3.55 open diff.
265s are close to 32" tall, depending on brand and tread. If you aren't towing much, that's probably fine with 4.10s. Acutally, you can still tow quite a bit, you just won't be able to cruise all the time in overdrive while towing. With enough miles, the fuel economy gain of being able to stay in OD most of the time will offset the cost of the gear change. But, it takes a LOT of miles towing a lot of weight.
My suggestion.... try it. Maybe even add a 5-star tuner which should reduce transmission hunting. Then, if that's not enough, or you want more grunt off-the-line, think about gears. But a tuner is MUCH cheaper, and will let you play with shift points and shift pressures. I'm extremely happy with my 4.56 gears, but honestly if I didn't tow the weights I do, it would be overkill.
I think going by Ford GCWR's is a good baseline to go by, since they establish primarily on performance anyways, and in my experience it makes a pretty decent real-world guide.
For the V10, Ford has the following GCWRs:
3.73 = 15,000 pounds
4.10 = 18,500
4.30 = 20,000
4.56 = 22,000
And even high, all the up to 30,000 with 6.13 gears.
An increase in tire size by 1.5" would reduce your effective max GCWR by 10% (since that would reduce the Radius by that much, which in turn reduces tractive force by the same amount).
Going over GCWR isn't the end of the world, but it will be working harder and therefore you'll use more fuel than if you stayed within specs, assuming everything is stock. Going from 3.73 gears to 4.56 gears gained me about 1.5 MPG towing, but I also lost about the same solo.